Regenerator for a Stirling engine

ABSTRACT

A regenerator for a thermal cycle engine and methods for its manufacture. The regenerator has a random network of fibers formed to fill a specified volume and a material for cross-linking the fibers at points of close contact between fibers of the network. A method for manufacturing a regenerator has steps of providing a length of knitted metal tape and wrapping a plurality of layers of the tape in an annular spiral.

[0001] The present application is a divisional of U.S. application Ser.No. 09/818,321, filed Mar. 27, 2001, Atty Dkt. 2229/122, itself acontinuation-in-part of U.S. application Ser. No. 09/517,245, filed Mar.2, 2000, itself a continuation-in-part application of U.S. applicationSer. No. 09/115,383, filed Jul. 14, 1998, and issued May 16, 2000 asU.S. Pat. No. 6,062,023, and a continuation-in-part also of Ser. No.09/115,381, filed Jul. 14, 1998 and now abandoned, each of which claimspriority from U.S. provisional application No. 60/052,535, filed Jul.15, 1997, all of which applications are herein incorporated byreference.

TECHNICAL FIELD

[0002] The present invention pertains to regenerators for a Stirlingcycle heat engine and their manufacture.

BACKGROUND OF THE INVENTION

[0003] Stirling cycle machines, including engines and refrigerators,have a long technological heritage, described in detail in Walker,Stirling Engines, Oxford University Press (1980), incorporated herein byreference. The principle underlying the Stirling cycle engine is themechanical realization of the Stirling thermodynamic cycle:isovolumetric heating of a gas within a cylinder, isothermal expansionof the gas (during which work is performed by driving a piston),isovolumetric cooling, and isothermal compression.

[0004] Additional background regarding aspects of Stirling cyclemachines and improvements thereto are discussed in Hargreaves, ThePhillips Stirling Engine (Elsevier, Amsterdam, 1991) and in co-pendingU.S. patent applications Ser. No. 09/115,383, filed Jul. 14, 1998, andSer. No. 09/115,381, filed Jul. 14, 1998, which reference and both ofwhich applications are herein incorporated by reference.

[0005] The principle of operation of a Stirling engine is readilydescribed with reference to FIGS. 1a-1 e, wherein identical numerals areused to identify the same or similar parts. Many mechanical layouts ofStirling cycle machines are known in the art, and the particularStirling engine designated generally by numeral 10 is shown merely forillustrative purposes. In FIGS. 1a to 1 d, piston 12 and a displacer 14move in phased reciprocating motion within cylinders 16 which, in someembodiments of the Stirling engine, may be a single cylinder. A workingfluid contained within cylinders 16 is constrained by seals fromescaping around piston 12 and displacer 14. The working fluid is chosenfor its thermodynamic properties, as discussed in the description below,and is typically helium at a pressure of several atmospheres. Theposition of displacer 14 governs whether the working fluid is in contactwith hot interface 18 or cold interface 20, corresponding, respectively,to the interfaces at which heat is supplied to and extracted from theworking fluid. The supply and extraction of heat is discussed in furtherdetail below. The volume of working fluid governed by the position ofthe piston 12 is referred to as compression space 22.

[0006] During the first phase of the engine cycle, the startingcondition of which is depicted in FIG. 1a, piston 12 compresses thefluid in compression space 22. The compression occurs at a substantiallyconstant temperature because heat is extracted from the fluid to theambient environment. The condition of engine 10 after compression isdepicted in FIG. 1b. During the second phase of the cycle, displacer 14moves in the direction of cold interface 20, with the working fluiddisplaced from the region of cold interface 20 to the region of hotinterface 18. This phase may be referred to as the transfer phase. Atthe end of the transfer phase, the fluid is at a higher pressure sincethe working fluid has been heated at constant volume. The increasedpressure is depicted symbolically in FIG. 1c by the reading of pressuregauge 24.

[0007] During the third phase (the expansion stroke) of the enginecycle, the volume of compression space 22 increases as heat is drawn infrom outside engine 10, thereby converting heat to work. In practice,heat is provided to the fluid by means of a heater head 100 (shown inFIG. 2) which is discussed in greater detail in the description below.At the end of the expansion phase, compression space 22 is full of coldfluid, as depicted in FIG. 1d. During the fourth phase of the enginecycle, fluid is transferred from the region of hot interface 18 to theregion of cold interface 20 by motion of displacer 14 in the opposingsense. At the end of this second transfer phase, the fluid fillscompression space 22 and cold interface 20, as depicted in FIG. 1a, andis ready for a repetition of the compression phase. The Stirling cycleis depicted in a P-V (pressure-volume) diagram as shown in FIG. 1e.

[0008] Additionally, on passing from the region of hot interface 18 tothe region of cold interface 20, the fluid may pass through aregenerator 134 (shown in FIG. 2). Regenerator 134 is a matrix ofmaterial having a large ratio of surface area to volume which serves toabsorb heat from the fluid when it enters hot from the region of hotinterface 18 and to heat the fluid when it passes from the region ofcold interface 20.

[0009] Stirling cycle engines have not generally been used in practicalapplications due to such practical considerations as efficiency,lifetime, and cost, which are addressed by the instant invention.

SUMMARY OF THE INVENTION

[0010] In accordance with preferred embodiments of the presentinvention, a method is provided for manufacturing a regenerator for athermal cycle engine. The method includes wrapping a plurality of layersof knitted metal tape in an annular spiral. The knitted metal tape maybe wrapped in parallel annular layers around a mandrel and then themandrel may be removed. Additionally, the knitted metal tape may beflattened.

[0011] In accordance with alternate embodiments of the invention, amethod for manufacturing a regenerator for a thermal cycle engine isprovided that includes axially compressing a length of knitted metaltube along the tube axis thereby generating a bellows.

[0012] In further embodiments of the invention, a regenerator isprovided for a thermal cycle engine. The regenerator has a randomnetwork of fibers formed to fill a specified volume and a material forcross-linking the fibers at points of close contact between fibers ofthe network. The fibers may be metal, or, more particularly, steel wool.The material for cross-linking the fibers may be nickel. The fibers mayalso be silica glass and the material for cross-linking the fibers maybe tetraethylorthosilicate.

[0013] In yet further embodiments of the invention, a regenerator isprovided for a thermal cycle engine, where the regenerator has a volumedefined by an inner sleeve and an outer sleeve, the inner and outersleeves being substantially concentric, and two parallel planes, eachsubstantially perpendicular to each of the inner and outer sleeves. Theregenerator also has a random network of fibers contained within thevolume, and two screens, each coupled to both the inner and outersleeves and lying in one of the two parallel planes, such as to containthe random network of fibers within the volume.

[0014] A regenerator for a thermal cycle engine may be manufactured, inaccordance with other embodiments of the invention, by filling a formwith a random network of electrically conducting fibers, immersing theform in an electroplating solution, and applying a current between thesolution and the random network of fibers in such a manner as to deposita material for cross-linking the electrically conducting fibers atpoints of close contact between fibers. Alternatively, a form may befilled with a random network of fibers, whereupon the random network offibers is sintered in such a manner as to cross-link the fibers atpoints of close contact between fibers.

[0015] A further method for manufacturing a regenerator for a thermalcycle engine, in accordance with embodiments of the invention, includesthe steps of forming a reticulated foam into a specified shape,depositing a ceramic slurry onto the reticulated foam, heat treating theslurry in such a manner as to burn off the foam, and sintering theceramic.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will be more readily understood by reference to thefollowing description, taken with the accompanying drawings, in which:

[0017]FIGS. 1a-1 e depict the principle of operation of a prior artStirling cycle machine;

[0018]FIG. 2 shows a side view in cross section of the heater head andcombustion chamber of a thermal engine in accordance with a preferredembodiment of the present invention;

[0019]FIG. 3a depicts the fabrication of a regenerator by electroplatingof a fibrilose starting material in accordance with an embodiment of thepresent invention;

[0020]FIG. 3b is a cross-sectional of the regenerator chamber of aStirling cycle engine in accordance with the present invention;

[0021]FIG. 4a depicts fabricating a knit-wire wound regenerator inaccordance with embodiments of the present invention;

[0022]FIG. 4b is a knit-wire tube prior to fabrication into a bellowsconfiguration;

[0023]FIG. 4c shows the knit-wire tube of FIG. 4a scored for compressionin accordance with an embodiment of the invention;

[0024]FIG. 4d shows the knit-wire tube of FIG. 4a scored in analternative manner for compression in accordance with an embodiment ofthe invention; and

[0025]FIG. 4e shows the knit-wire tube of FIG. 4a after compression toform a bellows regenerator in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] Referring to FIG. 2, a cross-sectional view is shown of theexpansion volume 98 of a thermal cycle engine, shown for illustrativepurposes as a Stirling cycle engine designated generally by numeral 96,and of the corresponding thermal control structures. Heater head 100 issubstantially a cylinder having one closed end 120 (otherwise referredto as the cylinder head) and an open end 118. Closed end 120 is disposedin a combustion chamber 122 defined by an inner combustor structure 110.Hot combustion gases in combustion chamber 122 are in direct thermalcontact with heater head 100 and thermal energy is transferred byconduction from the combustion gases to the heater head and from theheater head to the working fluid of the thermal engine, typicallyhelium. Other gases such as nitrogen, for example, may be used withinthe scope of the present invention, with a preferable working fluidhaving high thermal conductivity and low viscosity. Non-combustiblegases are also preferred. Heat is transferred from the combustion gasesto the heater head as the combustion gases flow along the outsidesurface of closed end 120 within a gas flow channel 113.

[0027] Expansion volume 98 is surrounded on its sides by expansioncylinder liner 115, disposed, in turn, inside heater head 100 andtypically supported by the heater head. The expansion piston 121 travelsalong the interior of expansion cylinder liner 115. As the expansionpiston travels toward closed end 120 of heater head 100, the workingfluid within the heater head is displaced and caused to flow throughflow channels defined by the outer surface of the expansion cylinderliner 115 and the inner surface of heater head 100.

[0028] As the working fluid is displaced from expansion cylinder 115 bythe expansion piston, working fluid is further heated in passage overthe inner pin array 124 and driven through regenerator chamber 132. Aregenerator 134 is used in a Stirling cycle machine, as discussed above,to add and remove heat from the working fluid during different phases ofthe Stirling cycle. The regenerator used in a Stirling cycle machinemust be capable of high heat transfer rates which typically suggests ahigh heat transfer area and low flow resistance to the working fluid.Low flow resistance also contributes to the overall efficiency of theengine by reducing the energy required to pump the working fluid.Additionally, regenerator 134 must be fabricated in such a manner as toresist spalling or fragmentation because fragments may be entrained inthe working fluid and transported to the compression or expansioncylinders and result in damage to the piston seals.

[0029] One regenerator design uses several hundred stacked metal screensWhile exhibiting a high heat transfer surface, low flow resistance andlow spalling, metal screens may suffer the disadvantage that theircutting and handling may generate small metal fragments that must beremoved before assembling the regenerator. Additionally, stainless steelwoven wire mesh contributes appreciably to the cost of the Stirlingcycle engine.

[0030] In accordance with an embodiment of the invention, a threedimensional random fiber network, such as stainless steel wool orceramic fiber, for example, may be used as the regenerator, as nowdescribed with reference to FIG. 3a. Stainless steel wool regenerator200 advantageously provides a large surface area to volume ratio,thereby providing favorable heat transfer rates at low fluid flowfriction in a compact form. Additionally, cumbersome manufacturing stepsof cutting, cleaning and assembling large numbers of screens areadvantageously eliminated. The low mechanical strength of steel wool andthe tendency of steel wool to spall may both be overcome as nowdescribed. In accordance with an embodiment of the invention, theindividual steel wires 202, 204 are “cross-linked” into a unitary 3Dwire matrix.

[0031] The starting material for the regenerator may be fibrilose and ofrandom fiber form such as either steel or nickel wool. The compositionof the fiber may be a glass or a ceramic or a metal such as steel,copper, or other high temperature materials. The diameter of the fiberis preferably in the range from 10 micrometers to 1 millimeter dependingon the size of the regenerator and the properties of the metal. Thestarting material is placed into a form corresponding to the final shapeof the regenerator which is depicted in cross-section in FIG. 3b. Innercanister cylindrical wall 220, outer canister cylindrical wall 222, andregenerator network 200 are shown. The density of the regenerator iscontrolled by the amount of starting material placed in the form. Theform may be porous to allow fluids to pass through the form.

[0032] In an alternate embodiment of the invention, unsintered steelwool is employed as regenerator network 200. Regenerator network 200 isthen retained within the regenerator canister by regenerator retainingscreens 224 or other filter, thereby comprising a “basket” which mayadvantageously capture steel wool fragments.

[0033] In one embodiment of the invention, applicable to startingmaterial that is electrically conducting, the starting material isplaced in a porous form and placed in an electrolyte bath. The startingmaterial may be a metal, such as stainless steel, for example. Anelectrical connection is made with the starting material thereby formingan electrode. Cross-linking of the individual fibers in the startingmaterial is accomplished by electrically depositing a second material206 onto the starting material. The selection of the starting materialwill depend on such factors as the particular deposition techniquechosen and the chemical compatibility of the first and second materials,as known to one of ordinary skill in the electrochemical art. Duringdeposition, the second material will build up on the starting materialand form bridges 208 between the individual fibers of the startingmaterial in places where the individual fibers are in close proximity toeach other. The deposition is continued until the bridges have grown toa sufficient size to hold the two individual fibers rigidly in place.

[0034] The deposition duration depends on the particular depositionprocess and is easily determined by one of ordinary skill in the art.After the deposition is completed, the regenerator is removed from thebath and the form and is cleaned.

[0035] In another embodiment of the invention, the starting material isplaced in a form that may be porous or not. The form containing thestarting material is placed in a furnace and is partially sintered intoa unitary piece. The selection of the sintering temperature andsintering time is easily determined by one of ordinary skill in thesintering art.

[0036] In another embodiment of the invention, the starting material isplaced in a porous form. The form containing the starting material isplaced in a chemical bath and a second material, such as nickel, ischemically deposited to form bridges between the individual fibers.

[0037] In another embodiment of the invention, the starting material isa silica glass fiber which is placed into a porous form. The glass fiberand form is dipped in a solution of tetraethylorthosilicate (TEOS) andethanol so that the fiber is completely wetted by the solution. Thefiber and form are removed from the solution and allowed to drain in ahumid atmosphere. The solution will form meniscoidal shapes bridgingfibers in close proximity to each other. The humidity of the atmospherewill start the hydrolysis-condensation reaction that converts the TEOSto silica forming a cross link between the two fibers. The fiber andform may be heat treated at a temperature less than 1000° C., mostpreferably less than 600° C., to remove the reactant products and form asilica bridge between the fibers.

[0038] In another embodiment of the invention, a ceramic slurry isdeposited onto a reticulated foam having the shape of the regenerator.The slurry is dried on the reticulated foam and heat treated to burn offthe foam and sinter the ceramic. The ceramic may be composed of an oxideceramic such as cordierite, alumina, or zirconia. The composition of theceramic slurry and the heat treatment profile is easily specified by oneof ordinary skill in the ceramic processing art.

[0039] In yet other embodiments of the invention, knit or woven wire isemployed in fabrication of a regenerator as now described with referenceto FIG. 4a. In accordance with these embodiments, knit or woven wiretube 201 is flattened by rollers 202 into tape 204, in which form it iswound about mandrel 206 into annular layers 208. Stainless steel isadvantageously used for knit wire tube 201 because of its ability towithstand elevated temperature operation, and the diameter of the wireused is typically in the range of 1-2 mils, however other materials andgauges may be used within the scope of the present invention.Alternatively, a plurality, typically 5-10, of the stainless steel wiresmay be loosely wound into a multi-filament thread prior to knitting intoa wire tube. This process advantageously strengthens the resulting tube201. When mandrel 206 is removed, annular assembly 210 may be used as aregenerator in a thermal cycle engine.

[0040] Still another embodiment of the invention is now described withreference to FIGS. 4b-4 e. Knit or woven wire tube 201, shown in itsright cylindrical form in FIG. 4b, is shown scored and partiallycompressed in FIG. 4c. Alternatively, the scoring may be at an angle 214with respect to the central axis 212 of the tube, as shown in FIG. 4d.Tube 201 is then axially compressed along central axis 212 to form thebellows form 216 shown in FIG. 4e that is then disposed as a regeneratorwithin the regenerator volume 132 (shown in FIG. 2) of a Stirling cycleengine.

[0041] The devices and methods described herein may be applied in otherapplications besides the Stirling engine in terms of which the inventionhas been described. The described embodiments of the invention areintended to be merely exemplary and numerous variations andmodifications will be apparent to those skilled in the art. All suchvariations and modifications are intended to be within the scope of thepresent invention as defined in the appended claims.

We claim:
 1. A regenerator for a thermal cycle engine, the regeneratorcomprising: a. a random network of fibers formed to fill a specifiedvolume; and b. a material for cross-linking the fibers at points ofclose contact between fibers of the network.
 2. A regenerator accordingto claim 1, wherein the fibers are metal.
 3. A regenerator according toclaim 1, wherein the fibers are chosen from the group of steel andnickel wool.
 4. A regenerator according to claim 1, wherein the materialfor cross-linking the fibers is nickel.
 5. A regenerator according toclaim 1, wherein the fibers are silica glass and the material forcross-linking the fibers is tetraethylorthosilicate.
 6. A regeneratorfor a thermal cycle engine, the regenerator comprising: a. a volumedefined by an inner sleeve and an outer sleeve, the inner and outersleeves being substantially concentric, and two parallel planes, eachsubstantially perpendicular to each of the inner and outer sleeves; b. arandom network of fibers contained within the volume; and c. a first anda second screen, each screen coupled to both the inner and outer sleevesand lying in one of the two parallel planes, such as to contain therandom network of fibers within the volume.
 7. A regenerator accordingto claim 6, wherein the fibers are chosen from the group of steel andnickel wool.
 8. A regenerator for a thermal cycle engine, theregenerator comprising: a. a random network of fibers formed to fill anannular volume; and b. a filter disposed at at least one end of theannular volume for retaining fragments within the annular volume.
 9. Aregenerator according to claim 8, wherein the filter is a screen.